Arsenic contamination of paddy rice fields:a study on geographic pattern in the rice growing areas of Punjab, Pakistan

Abstract

The Punjab plains of Pakistan are a major rice growing area where irrigation with groundwater is a potential concern because Arsenic (As) accumulating in soil can reduce rice yield and translocate to the rice grain. To address this issue, matched sets of irrigation water, paddy soil, and rice grains from 60 fields across the region were analyzed. Concentrations of As in irrigation water (65+32 μg/L, n=20) are higher in the floodplain of the Ravi River compared to the floodplains of the Chenab (13+9 μg/L; n=9) and Jhelum (4+5 μg/L, n=6) rivers, as well as the intervening Rechna (6+6 μg/L, n=19) and Chaj doabs (0.8+0.2 μg/L). Based on 10 surface samples collected diagonally across each rice field, area-weighted soil As concentrations range from 8-21 mg/kg along the Ravi River, 7-12 and 6-11 mg/kg along the Chenab and Jhelum rivers, respectively, and 6-7 mg/kg within the Rachna and <6 in Chaj doab. The pattern of As accumulation is consistent with farmers supplementing rainfall with groundwater to grow rice in recent decades. In contrast to irrigation water and paddy soil, the As content of polished grains export-quality basmati rice of 0.09+0.05 mg/kg (n=60 sets of triplicates) is remarkably low across the entire area. The Na-Cl type of water is dominant in Doabs (Rechna and Chaj) with low As concentrations whereas Ca-HCO3 type of water is dominant in Ravi flood plain with high As concentrations. The average Hazard quotients (HQ) value of 9.09E+00 and Potential Cancer Risk (CR) value of 4.09E-03 in Ravi flood plain indicates the potential health risk to the consumers of well water for drinking purposes and also long term irrigation with this water will result As accumulation in paddy soil. Based on average values, the Sodium Adsorption Ratio (SAR), Magnesium Hazard (MH), %Sodium (%Na) and Kelly Ratio (KR) follows the sequence Rechna Doab>Chaj Doab>Chenab floodplain>Jhelum floodplain>Ravi floodplain in 5 geographic units, suggesting low salinity water in floodplains and saline water in Doabs. The principal component analysis (PCA) and correlation matrices shows the contribution of both natural and anthropogenic activities. The Chenab>Chaj>Ravi>Jhelum>Rechna sequence of geographic units based on average soil Fe content does not follows the area-weighted soil mean As i,e Ravi>Chenab>Rachna>Jhelum>Chaj sequence based on the areas-weighted soil As. Arsenic contamination of paddy rice fields: a study on geographic pattern in the rice growing areas of Punjab, Pakistan Page xvi The Mn in soil follows the sequence Ravi>Chenab>Jhelum>Chaj>Rachna in 5 geographic units based on average soil Mn but it is evident that Mn concentrations in soil shows a little variations unlike soil As which are clearly high in Ravi river flood plain and follows a geographic sequence i,e Ravi>Chenab>Rachna>Jhelum>Chaj sequence based on the areas-weighted soil As. The OM% in soil follows the sequence Ravi>Jhelum>Chenab>Chaj>Rachna in 5 geographic units based on average soil OM% but show that the use of OM is high in Ravi river flood plain compared to other flood plains and doabs. The overall average of 0.09 mg/kg in total rice As for the region is consistent with 5 previous studies of basmati rice from Pakistan and India compared to other varieties from India and Bangladesh which shows higher average As content of rice, with even a few studies showing averages exceeding 0.5 mg/kg As. Therefore, basmati rice appears to the major reason of low As despite of high As in water and soil. Using matched water, soil, and rice As measurements from a considerable number of sites distributed across the Punjab plains of Pakistan, this study confirms that groundwater pumping is the main contributor of As concentrations in some paddy soil of Punjab. The contamination of paddy soil with As is currently limited in the region but is likely to increase in severity and geographic extent over time. The accumulation of As in paddy soil of the Punjab plains is not linked to a measurable increase in the uptake of As in rice grains of basmati rice. After reducing exposure from drinking well water along the Ravi River, the main concern in the region could potentially become the loss in rice yield resulting from further accumulation of As in soil over time. The present study also compares laboratory measurements of the As content of irrigation water and paddy soil with field measurements using the ITS Econo-Quick arsenic kit to show that farmers throughout South Asia could use this kit to determine to what extent their field and crop are at risk of contamination. This is why the ITS Econo Quick arsenic kit could be useful to farmers by helping them determine the As content of both their irrigation water and their paddy soil